In a grand manor house in northern Lebanon, eight men, all of them well-to-do professionals, had gathered to hear an important visitor talk about the war.

Their guest was late; the going had been heavy across the mountain from Beirut, north up the Bekaa valley and finally west along the flat, spotless, Iranian-made highway that leads to Hermel. Arriving from the bitter chill of a winter evening, he eased into a warm living room where the expectant men edged forward, addressing the new arrival by a nom de guerre widely known throughout Hezbollah, the powerful militant group he had joined more than 20 years ago.

"This is a war not just against us, but against humanity," he said. "And it is one that we will win."

He was referring to the war to the east in Syria, a conflict in which Hezbollah has admitted playing a significant role, rallying to the cause of the Syrian army in its protracted battle against the opposition forces and Sunni Islamist groupings ranged against it.

Speaking carefully and deliberately, the commander, whom the Guardian agreed not to name, initially stuck to the official script that characterises Hezbollah as reluctant saviours of a beleaguered nation hemmed in by extremist Sunni militants on one side and by Israel on the other. In nearly three years of insurrection and war in Syria, it has been difficult to hear anything else from a Hezbollah official.

But over two increasingly unguarded hours, the commander strayed on to themes rarely covered: the regional impact of the group's role in Syria, the intensity of the fighting and the performance of the Syrian Army, which not long ago had been fighting a losing battle to retain control of the country. Those foregathered listened intently. All broadly supported the fight against the Syrian opposition, even if they differed on the virtues of Syria's leader, Bashar al-Assad.

"They fight well. It is not fair to them to say that they are not taking the lead," he said of the battle-worn military regime. "They are there and they are fighting. They have lost 30,000 men. That is not an army that isn't fighting. We are there giving advice and in some cases tactical leadership. We do not take a lead role."

Fifteen kilometres north-east, the ruins of the Syrian border town of Qusayr tell a different story. In May, Hezbollah stormed the town from the south, achieving in three weeks what the army it supports had been unable to do in two years. Syrian tanks and troops took blocking positions to the north and east. The attack is believed to be the biggest co-ordinated engagement ever fought by the Iranian-backed, exclusively Shia Islamic militia, which is well-attuned to guerrilla warfare, but less so to a full frontal assault on a fortified urban centre.

The Qusayr battle cost Hezbollah 112 men. It was, however, defining for a different reason: it marked the first time that the group's leader, Hassan Nasrallah, had been prepared to reveal that his members were indeed fighting in Syria. The acknowledgment was perceived by many Sunni Arab leaders as an act of belligerence that poured fuel on the sectarian fire. In the eyes of Saudi Arabia and many of the Gulf states, the troika of Iran, Hezbollah and the Alawite-led Assad regime are no longer shy about taking the fight to Sunni Muslims in the name of regional hegemony.

"It is not like that at all," the commander said. "They are the majority and they think they are the victims. Aren't minorities supposed to be the vulnerable ones? We are defending our lands. We are defending our interests. If the takfiris [fundamentalist Sunni Islamists] had not started attacking the border Shia villages, we would not have been forced to act."

Throughout the discussion, the commander labelled all members of the opposition as takfiris. Pressed on whether he believed any opposition fighters remained committed to the uprising's original goals of reorienting power within Syria's current borders, he said: "If there were any mainstream revolutionaries back then, there are very few now.

"In [Sunni] history Ibn Tarmeyah spoke out three times against us. We have known what we have been up against for a long time."

The pertinence of ancient teachings to a here-and-now battle is a common theme on both sides of a now bitter and protracted divide which is steadily becoming the most serious schism between the two Islamic sects since a seminal dispute over who should succeed the prophet Muhammad nearly 1,400 years ago.

Increasing numbers on both sides – the almost exclusively Sunni opposition and the largely Shia-aligned interests of the regime – frame the war as a prelude to an apocalyptic showdown with a preordained foe. To the Hezbollah leader, the role of the group is underwritten by Islamic teachings, just as much as it is dictated by modern strategic realities.

"The battle is intense. The takfiris are committed. They want to destroy Syria and we will not let them."

Despite murmurings of unease in parts of Lebanon's Shia heartland, he said Hezbollah and its supporters resolutely supported the group's involvement in the war. "It is an extension of the ongoing war [with Israel]," he said. "The enemy wears a new cloth. They may not be doing all of this themselves, but their interests are being served."

Asked why it had taken more than two years for Nasrallah to acknowledge the group's intervention, he said: "There was a process needed. People are absolutely committed to the reality now because they know it is one and the same hand.

"We started around the Sayeda Zainab mosque [a revered Shia shrine near Damascus], then moved to the border villages, then Qusayr. There are members fighting throughout the country, but not in huge numbers."

Over the past three weeks, some Hezbollah members have been stationed on the outskirts of Aleppo, along with members of the Iranian military and a large contingent of a Shia militia, Abu al-Fadl al-Abbas, drawn mainly from Iraqi volunteers. Hezbollah is also playing a lead role in the Qalamoun mountains north-west of Damascus – a battle that if won would allow the victor access from the capital to Syria's third city, Homs.

On the other side are a mix of Syrians fighting to oust Assad and replace him with another leader, and jihadists who see the insurrection as means of re-establishing a caliphate in the area and a fundamentalist Islamic society that reflects the seventh-century life of the prophet Muhammad.

Fighting in both areas has been intense over the past week, with more than 420 people reported to have been killed in eastern Aleppo. Helicopters dropping improvised explosives are responsible for much of the carnage and medics in Aleppo report that large numbers of civilians are among the casualties. In Qalamoun, the battle is being fought in mountains and valleys, the type of terrain in which Hezbollah has trained for more than three decades.

The conversation broadens. These men are at home here. The founding parade that formed the organisation was held not far from this spot in 1982. Street posts throughout Hermel are festooned with fading posters of men who have died in Hezbollah's short, bloody history. All are revered as martyrs in the organisation's heartland. Many had died in battles past, fought against a traditional foe, Israel. But new vivid photos of young fresh-faced men and boys jut from among them. They instead perished in Syria, fighting other Muslims. And the Hezbollah leader had commanded many of them.

A maid made coffee runs from a spotless kitchen to the right of the group and a portrait of a smiling Bashar al-Assad, his late father Hafez, and Hassan Nasrallah overlooked them from a facing wall. The conversation turned to the role of the US in the region and its rapproachment with Iran. "They seem to be framing their foreign policy [in the Middle East] solely through the view of protecting Israel," he said. "There are, of course oil and gas interests, especially with Iran and Pakistan. But the discussions with Iran are welcomed. It is a step forward."

As for the old foe, Israel, he said: "None of their borders are safe now and this is not a good thing for them. They cannot be happy with the momentum anywhere in the region, especially Syria. Egypt is perhaps the only border that gives them comfort. The rest are outside of their control."

Solar-powered lamps provided by Iran light the pristine road back to the Bekaa Valley, where potholes and darkness replace the stretch of bitumen. Here, only 15km from one of Hezbollah's main strongholds, the world view also changes suddenly.

At the first Lebanese army checkpoint heading south, several soldiers stopped our car and asked if we had a place for one of them. "I'm deserting," a 19-year-old Sunni Muslim conscript said. "I've had enough of this. Another rocket [from Syria] just landed. I want to go there and fight."

Echoing the words of the Hezbollah leader, he said: "This is a war that we cannot lose. We will win, whatever the cost."